The all-seeing eye is a human being descended from ancient kings and queens since before ancient Egypt. The inheritance was enormous. Wealth beyond imagination.
Over several thousand years, that family line was kept to a single living male. Father was killed upon birth of a son. The Illuminati is a band a thieves who maintain the secret.
The Illuminati are insane. They believe themselves to be immortal and believe in a hodge-podge of Satan-like religions.
The All-Seeing Eye is a human. A good one. Benevolent and wise. Held in poverty and obscurity by the Illuminati thieves.
Why do they keep him alive? The Illuminati are not all that intelligent, or educated, so use the All-Seeing Eye's education, wisdom, and intelligence to present themselves as such. Essentially impersonating. Others see them as wise and powerful. They are charlatan thieves.